/Engine/Source/Runtime/Engine/Classes/Engine/GameInstance.h
Called as soon as the game mode is spawned, to allow additional PIE setting validation prior to creating the local players / etc... (called on pure clients too, in which case the game mode is nullptr)
virtual [FGameInstancePIEResult](API\Runtime\Engine\Engine\FGameInstancePIEResult) PostCreateGameModeForPIE ( const [FGameInstancePIEParameters](API\Runtime\Engine\Engine\FGameInstancePIEParameters) & Params, [AGameModeBase](API\Runtime\Engine\GameFramework\AGameModeBase) * GameMode )